As the critical dimensions of the elements that make up integrated circuits approach, and grow smaller than, the wavelength of the radiation used to form photolithographic images, various strategies have been developed to deal with the problem of how to continue resolving these elements from one another in the final images. One of the most successful approaches to the problem has been phase shifting masks.
The basic notion behind such masks is to eliminate or reduce the diffraction fringes that are generated immediately alongside any opaque edge. Since these fringes arise from Huygen's wavelets being alternately in and out of phase, they can be changed, and thus reduced, if the phase of the light in the immediate vicinity of an edge is changed. Thus, in the alternating phase shift mask (APSM), the phase of light that passes just outside the edge of an element that is to be imaged is shifted by 180 degrees and the diffraction fringe that would normally be there is eliminated.
In FIG. 1 we illustrate how, in the prior art, an APSM is formed for a single element 11 in the shape of a rectangle or a circle, such as might be intended to serve as a contact hole within an integrated circuit. The first step (symbolized by arrow 13) is to surround 11 with a ring of ‘dummy’ elements such as 12. This includes the placement of elements, such as 16, in the corners. The third step (arrow 14) is to assign phases (either zero or 180 degrees) for the light that will emerge after passing through elements 12. This is done by assigning phases in checker board fashion, as seen in the final arrangement 15, where the two phase types are symbolized by light and dark squares.
It should be noted that the way FIG. 1, and subsequent similar figures, are drawn is intended to depict transparent elements on an opaque background (necessitating the use of positive resists during photolithography), so neighboring elements transmit the same amount of light (i.e. approximately 100%) but the phases of the emerging beams differ by 180 degrees. It is also important to note that, in this scheme, the phase assigned to any given element is arbitrary just so long as the phases have been assigned in a checker board fashion.
The prior art procedure for designing masks for a densely packed set of elements is illustrated in FIG. 2. Starting with array of elements 21 (nine in this example), a ring of ‘dummy’ elements such as 22 is placed to surround the original nine elements. As before, this includes elements such as 26 in the corners. Also as before, phases are assigned to the final pattern 25 in a checker board fashion.
While these prior art algorithms work well enough for the simple examples used to illustrate them, several problems arise when they are applied to the more complex distributions of elements that are to be found in real circuit layouts. For example, in FIG. 3 we show typical layout 31 of fourteen contact holes. Following the procedures described above, the array has been expanded to become array 32 which contains fifty four elements, to all of which phases must now be assigned. When this procedure is followed for a full wafer layout, the number of required phase assignments can become quite large.
In order to reduce the number of required phase assignments, the prior art has also been using an alternative algorithm which is illustrated in FIG. 4. In this scheme, phases are assigned to the original elements first (arrow 44). Only then are the dummy elements added (arrow 43). The latter are then assigned alternating phases (as before). With this scheme, however, a problem can arise in the form of a conflict between the earlier assigned phases 46 and the later assigned phases 47. As can be seen in the rightmost array shown in FIG. 4, for the third and fourth rows from the bottom, the checkerboard assignment could not be maintained.
The present invention describes an array, and method for its formation, that requires both fewer phase assignment decisions to be made and is free from possible phase conflicts.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,249,904 B1, Cobb shows a process to correct edge placement distortion while Lin describes a double alternating PSM in U.S. Pat. No. 6,057,064. Travis et al., in U.S. Pat. No. 6,396,158 B1, show a mask process including assist features. U.S. Pat. No. 6,312,856 B1 (Lin) also reveals a PSM with assist features.